Re: Student demonstration in Magnetism

["John S. Denker" <jsd@MONMOUTH.COM>]

Tina Fanetti wrote:
> One of my students would like to do a demonstration in
>  physics class.

Good.

> He wants to show how you can magnetically levitate a wire.

OK.

>  I have no idea how to do this.  I can solve the formulas
> and get numbers.  He has done this and has come up with
> needing a current of 99 A.  I don't think so.

It depends on what you mean by "need".

For a big fat wire in a wimpy field, you might need 99 amps
or even 999 amps.  But this is a highly conditional need.

So use a smaller wire.  The current-carrying capacity goes like
cross-sectional area.  But the mass per unit length goes also
like cross-sectional area.   So if you can levitate a fat wire
using 99 amps, you can levitate a skinnier wire with a more-
reasonable current.

It also depends on the magnetic field strength.  Using a weak
field (such as the geomagnetic field) any ordinary wire will
burn itself up before levitating.  But in a strong field, you
can produce more than enough force for levitation.  Think of
the voice-coil in a speaker:  it accelerates at many, many Gees.
Levitation requires only one Gee.